net: TestListenerClose flaky on dragonfly

[bradfitz]

Bunch of these lately:

http://build.golang.org/log/1cb49fcb72f133c814d420106795a4aca11607b6

--- FAIL: TestListenerClose (0.00s)
    net_test.go:227: Dial to closed TCP listener succeeeded.
FAIL
FAIL    net 1.891s

/cc @mikioh

[gopherbot]

CL https://golang.org/cl/18959 mentions this issue.

[mikioh]

Note that this is a regression in Go 1.6, which is introduced by 19aa420.